I would suggest checking:
1. Shocks (bounce the car, more than one bounce = bad).
2. Check your tie rods (if you can twist them by hand = bad)
Things that will improve it the best would be:
1. High performance shocks (KYB's would be the cheapest and work just fine).
2. Bigger sway bar 1" is good.
3. The high performance steering box you have. (This will be really important if you have a lot of wear in your original one). A simple way of checking this wear is to, with the car not running and sitting on the ground, move the steering wheel back and forth. If the wheels move with your movements, and you don't move too far, it is good. The new box will still help though.
4. Polyurethane bushings.
I know others will have input on this, but this is atleast a start. If you can do the work yourself, and then take the car out for an alignment, I think the cost will come out below your $600 dollar mark.
67 Firebird Coupe--"Frankenstein"
94 LT1 engine, 4L60E transmission. Aluminum radiator, Tunercat, 160 T-Stat, Iroc-Z Steering box, GTA 16x8" wheels 245/50/r16 front and rear, 8.5" posi rear end 3.08 gears(I know, will eventually put in 3.73 gears), Guldstrand mod, poly bushings on front/rear suspension plus engine/transmission mounts, solid aluminum body bushings.
Definition of Project car!